Comparisons With Other Eruptions

The May 18, 1980, eruption of Mount St. Helens was exceeded in "size"
by many other eruptions, both in historic times and in the recent geologic
past.

For the study of earthquakes, two standard measures of the "size"
of the seismic event are commonly used: the Richter Magnitude Scale (based
on energy released as measured by seismometers) and the Modified Mercalli
Intensity Scale (based on damage caused as assessed by people). Although
some attempts have been made to develop a scale to compare the relative
sizes of volcanic eruptions, none has yet been adopted for general use.
Volcanologists have proposed and used various schemes to rank eruptions,
and these generally included one or more of the following factors--height
of eruption column, volume of material erupted, distance and height of hurled
blocks and fragments, amount of aerosols injected into the upper atmosphere,
and duration of eruption. All these factors relate directly or indirectly
to the total amount of energy released during the eruption. Perhaps the
two most commonly used and directly measurable factors are eruption volume
and height of the eruption column.

The May 18 eruption ejected about 0.3 cubic mile of uncompacted ash,
not counting an unknown but probably much smaller amount that was deposited
in the atmosphere or too diffuse to form measurable deposits. This volume
of ash is less than those of several earlier eruptions of Mount St. Helens
and considerably less than the ejecta volumes of some historic eruptions
elsewhere. The 1815 eruption of Tambora (Sumbawa, Indonesia) ejected about
30 to 80 times more ash than did Mount St. Helens in 1980. The 1815 Tambora
eruption ranks as the largest known explosive eruption in historic times.
But even the Tambora eruption pales by comparison with the gigantic pyroclastic
eruptions from volcanic systems such as Long Valley Caldera (California),
Valles Caldera (New Mexico), and Yellowstone Caldera (Wyoming)--which, within
about the last million years, produced ejecta volumes as much as 100 times
greater.

Ejecta volume, in cubic miles

Some scientists recently proposed the Volcanic Explosivity Index (VEI)
to attempt to standardize the assignment of the size of an explosive eruption,
using ejecta volume as well as the other criteria mentioned earlier. The
VEI scale ranges from 0 to 8. A VEI of 0 denotes a nonexplosive eruption,
regardless of volume of erupted products. Eruptions designated a VEI of
5 or higher are considered "very large" explosive events, which
occur worldwide only on an average of about once every 2 decades. The May
1980 eruption of Mount St. Helens rated a VEI of 5, but just barely; its
lateral blast was powerful, but its output of magma was rather small. The
VEI has been determined for more than 5,000 eruptions in the last 10,000
years. None of these eruptions rates the maximum VEI of 8. For example,
the eruption of Vesuvius Volcano in A.D. 79, which destroyed Pompeii and
Herculaneum, only rates a VEI of 5. Since A.D. 1500, only 21 eruptions with
VEI 5 or greater have occurred: one VEI 7 (the 1815 Tambora eruption), four
of VEI 6 (including Krakatau in 1883), and sixteen of VEI 5 (counting Mount
St. Helens in 1989 and El Chichon, Mexico in 1982). Considered barely "very
large," the eruption of Mount St. Helens in May 1980 was smaller than
most other "very large" eruptions within the past 10,000 years
and much smaller than the enormous caldera-forming eruptions--which would
rate VEl's of 8--that took place earlier than 10,000 years ago.

The number of casualties and extent of destruction also have been used
to compare the "bigness" of volcanic eruptions. For obvious reasons,
such comparisons are limited at best and misleading at worst. Some of the
most destructive eruptions have not been in other terms "very large."
For example, mudflows triggered by the November 1985 eruption of Nevado
del Ruiz (Colombia) killed more than 25,000 people--resulting in the worst
volcanic disaster in the 20th century since the catastrophe at Mont Pelee
in 1902. Yet, the eruption was very small, producing only about 3 percent
of the volume of ash ejected during the May 1980 eruption of Mount St. Helens.
As the table below clearly shows, of the seven greatest volcanic disasters
in terms of casualties since A.D. 1500, only two of them (Tambora and Krakatau)
qualify as "very large" eruptions (VEl's greater than 5) in terms
of their explosive force.

The May 1980 eruption of Mount St. Helens has a higher VEI (5) than five
of the deadliest eruptions in the history of mankind, but it resulted in
the loss of far fewer lives (57). Loss of life would have been much greater
if a hazard warning had not been issued and a zone of restricted access
had not been established.